The present invention relates to an ATM (Asynchronous Transfer mode) transmission system for transmitting an ATM cell through a radio channel or a wired channel, in particular, relates to such a system which uses a communication channel with high efficiency and suppresses useless traffic.
In an ATM transmission system, a data is transmitted by using an ATM cell which has a fixed length of data. An ATM cell has generally 53 bytes data including a cell header having 5 bytes followed by an information field having 48 bytes, as shown in FIG. 5.
Since an ATM cell has fixed length, high speed data transmission is possible by repetition of relatively simple process.
In an ATM transmission system, an ATM cell is repeated by a node which is called an ATM switch so that the ATM cell is transmitted from a source terminal equipment to a destination terminal equipment. Assuming that an ATM switch has the function to discard a wrong ATM cell (as shown in Japanese patent laid open publication 163141/1996), when there is something wrong in an ATM cell, such as a queue of an ATM cell and/or overflow of a buffer memory, a useless traffic may be suppressed for a transmission line at an output side of the ATM switch, however, no transmission is suppressed in a transmission line at an input side of the ATM switch.
Further, in a prior art, the discard of an ATM cell is triggered by a long queue of an ATM cell in an ATM switch and an overflow of a buffer memory, but not by a transmission error of an ATM cell. A transmission error occurs with high probability in case of a wireless transmission line.
FIG. 11 shows a basic system structure of a wireless ATM system. In FIG. 11, the numeral 11 shows a base station, 12, 13 and 14 are a radio module, 15, 16 and 17 are a terminal equipment, and 18 shows a network.
In FIG. 11, the base station 11 is coupled with radio modules 12-14 through a wireless channel. Each of radio modules 12-14 is connected to a related terminal equipment 15-17. Further, the base station 11 is coupled with the network 18 through an optical fiber, or a metal cable.
The terminal equipment 15-17 and the network 18 carry out transmission and reception of data by using an ATM cell, which is the minimum unit of data to be transmitted.
FIG. 12 shows a protocol stack concerning a user plane (U plane) of a wireless ATM system of FIG. 11. The base station 11 is coupled with the network 18 through a node 19.
As shown in FIG. 12, the protocol of the terminal equipment 15-17 comprises a physical layer (PHY), an ATM layer (ATM), an ATM adaptation layer (AAL), and an application layer (AP). Each of the radio modules 12-14 and the base station 11 has the protocol comprising a physical layer (PHY), an ATM layer (ATM), a wireless layer (Wireless), a data link layer (DLC), and a media access control layer (MAC). The node 19 has a physical layer (PHY) and an ATM layer (ATM).
As for a U plane of the base station 11 and the radio modules 12-14, a layer higher than an ATM adaptation layer (AAL) is not terminated. The terminal equipment 15-17 are seamlessly coupled with the node 19 in the network 18 through the ATM layer (ATM). In a wireless channel between the base station 11 and the radio modules 12-14, a plurality of virtual paths (VP) and virtual channels (VC) which are a logical link of an ATM layer may exist.
FIG. 13 shows a flow of an ATM cell C1-C8 when a terminal equipment 15 transmits a network 18 a data, in a wireless ATM system in FIG. 11. In the embodiment of FIG. 13, a radio module 12 is a transmitter, a base station 11 is a receiver, and the terminal 15 is sending an ATM cell.
As shown in FIG. 13, it is assumed that the terminal 15 sends ATM cells C1 through C8 sequentially to the radio module 12, and that the second ATM cell C2 is erroneous in the wireless section between the radio module 12 and the base station 11, so that the base station 11 receives the first ATM cell C1 and the ATM cells C3 through C8. Then, the base station 11 sends the first ATM cell C1 and the cells C3 through C8 to the network 18.
A base station 11 and a radio module 12 have a transmitting station and a receiving station as shown in FIG. 14. A transmitting station comprises an input means for accepting an ATM cell, a transmit buffer, and a transmit means. The transmit buffer stores temporarily an ATM cell which is in the queue for transmission. A receiving station comprises a receive means, a receive buffer, and an output means. An ATM cell received by the receive means is stored temporarily in the receive buffer. An ATM cell is read out of the receive buffer when requested, and is transmitted through the output means.
FIG. 15 shows the transfer sequence of the ATM cells C1 through C8 in the wireless section in case of FIG. 13. In FIG. 15, a TDMA (Time Division Multiple Access) system is used in wireless data communication between a radio module 12 and a base station 11, and each TDMA frame includes three ATM cells.
In FIG. 15, when cells in a first TDMA frame are transmitted, a first through third ATM cells C1-C3 are transmitted. It is assumed that a second ATM cell C2 is erroneous in a wireless section. Therefore, the base station 11 receives only the first ATM cell C1 and the third ATM cell C3.
When the ATM cell in the first TDMA frame has been transmitted, the radio module 12 has five ATM cells C4, C5, C6, C7 and C8 in the transmit buffer. The fourth through sixth ATM cells C4-C6 are transmitted when the cells in the second TDMA frame is transmitted. Then, the base station 11 has five ATM cells C1, C3, C4, C5 and C6 in the receive buffer.
When the ATM cells in the second TDMA frame has been transmitted, the radio module 12 has two ATM cells C7 and C8 in the transmit buffer. Those two cells C7 and C8 are transmitted when the ATM cells in the third TDMA frame are transmitted.
As a result, the base station 11 receives seven ATM cells C1, C3, C4, C5, C6, C7 and C8. Then, the base station transmits the network 18 seven ATM cells C1, C3, C4, C5, C6, C7 and C8.
By the way, the recent network which uses an ATM communication system, can provide a variety of services. And, for instance, it is requested that a user terminal is seamlessly coupled with an ATM network including wireless means, in other words, it is requested that a user terminal is coupled with a network only through an ATM layer.
However, it should be noted that an ATM cell is erroneous with relatively high probability in a wireless communication section.
In a data communication service which uses an ATM system, a TCP/IP protocol is used in a higher layer. The communication using the TCP/IP protocol is implemented for instance by a standardized specification (IP over ATM; RFC 1483). In this case, AAL type 5 is used as an ATM adaptation layer (AAL).
In a data communication system using the AAL type 5 as the ATM adaptation layer, the size of a packet in a higher layer is generally larger than the data size (48 bytes) of an information field of an ATM cell, therefore, a packet in a higher layer is segmented to a plurality of ATM cells for transmission.
If an ATM cell or a plurality of ATM cells are in error in transmission, and an error correction by a destination terminal (forward error correction; FEC) can not recover a packet data, when a packet is transmitted in a plurality of ATM cells, then, a packet itself is in error, and it is dealt as a useless packet.
When re-transmission is carried out in a higher layer, the re-transmission of a whole packet must be carried out when only one ATM cell is erroneous. In this case, when a wireless section exists in a network, the probability of a non-useful packet might be relatively high, since the probability of an error of an ATM cell in a wireless section is high.
As described above, when an erroneous ATM cell is included in a packet which has a plurality of ATM cells, a whole packet must be discarded as a non-useless packet. Therefore, even correct ATM cells included in an erroneous packet might be discarded. The transmission of correct ATM cells which are to be discarded because of an erroneous ATM cell in the packet decreases the traffic efficiency in a communication line.
It is an object of the present invention to provide a new and improved ATM transmission system by overcoming the limitations and disadvantages of a prior ATM transmission system.
It is also an object of the present invention to provide an ATM transmission system which suppresses useless traffic.
The above and other objects are attained by an ATM transmission system comprising a transmitting station, a receiving station, a communication channel between said stations for transmitting an ATM cell, data communication with a protocol data unit (PDU) having a packet and a header being carried out by using an ATM adaptation layer (AAL) which can recognize an end of packet cell (EOP cell) in a PDU by referring to a header in said EOP cell, and a plurality of ATM cells which form said protocol data unit being continuously applied to said transmitting station; wherein said receiving station comprises; an error detection means for detection whether an ATM cell is received correctly or incorrectly or lost; a cell arrival informing means for informing the transmitting station the result of said detection; said transmitting station comprises; a transmit buffer for temporarily storing an ATM cell to be transmitted; a group handling means for handling a plurality of ATM cells which form a protocol data unit in a common convergence sublayer as one group; a receiving means of a said cell arrival information whether the ATM cell has been received correctly or incorrectly or lost from an associated receiving station; a group data discard means for discarding all the ATM cells belonging to the group which includes an incorrectly received or lost ATM cell stored in said transmit buffer, when said receiving means receives the information of an incorrectly received or lost ATM cell; an end of packet cell (EOP cell) transmitting means for transmitting an EOP cell which has a flag in a payload type field in an ATM cell header, said flag indicating that the EOP cell is a final cell in the protocol data unit of the group, when said group data discard means discards the ATM cell in said transmit buffer; an ATM cell discard means for discarding an ATM cell which belongs to the discarded group, and arrives after discard.
Preferably, said transmitting station further comprises means for attaching sequence number to each ATM cell to be transmitted, and said error detection means in said receiving station detects whether a cell is received or lost by checking a sequence number of a received ATM cell.
Preferably, a communication channel between a transmitting station and a receiving station includes a plurality of virtual channels; each virtual channel transmits ATM cells having different virtual path identifier and different virtual channel identifier; a transmitting station and a receiving station establish a logical transmission link for each virtual channel of an ATM layer; and said group handling means handles transmission and reception of an ATM cell for each established logical transmission link independently.
Preferably, a communication channel between a transmitting station and a receiving station includes a plurality of virtual channels; each virtual channel transmits ATM cells having different virtual path identifier and different virtual channel identifier; said receiving station comprises a sequence number informing means for informing sequence number of an ATM cell which is lost or wrongly received, to said transmitting station; said transmitting station comprises a table having relations between sequence number of an ATM cell and a group which said ATM cell belongs, and a group identify means for identifying a group which a wrongly received or lost ATM cell belongs, according to a sequence number informed by said receiving means and content of said table.
According to the present invention, in case there is one virtual channel, or a plurality of virtual channels, on a circuit, when an ATM cell which is a part of a protocol data unit (PDU) is consecutively applied to a transmitting station, the transmitting station decides an ATM cell which would be useless, and discards the same.